Method of treating cyanide-containing liquors in surface treatment installations

ABSTRACT

A METHOD OF REMOVING CYANIDE WASTES FROM CYANIDECONTAINING SURFACE TREATING BATHS IN A METAL-TREATING PROCESS WHICH INCLUDES AN ELECTROLYTIC ALKALINE CLEANING BATH AND A CYANIDE-CONTAINING SURFACE TREATING BATH COMPRISING PASSING THE WASTE CYANIDE-CONTAINING WATER FROM THE CYANIDE TREATMENT BATH TO AN ALKALINE ELECTRLYTIC CLEANING BATH, WHEREBY THE CYANIDE IS ELECTROLYTICALLY OXIDIZED.

Feb. 29, 1972 mcso EI'AL 3,645,857

METHOD OF TREATING GYANIDE-CONTAINING LIQUORS IN SURFACE TREATMENTINSTALLATIONS Filed June 19, 1969 United States Patent US. Cl. 204130Claim ABSTRACT OF THE DISCLOSURE A method of removing cyanide wastesfrom cyanidecontaining surface treating baths in a metal-treatingprocess which includes an electrolytic alkaline cleaning bath and acyanide-containing surface treating bath comprising passing the wastecyanide-containing water from the cyanide treatment bath to an alkalineelectrolytic cleaning bath, whereby the cyanide is electrolyticallyoxidized.

The present invention is concerned with a method of preventing thedischarge of cyanide-containing liquors from chemical metal surfacetreating processes wherein aqueous cyanide solutions are used. Suchcyanide-containing aqueous baths are used extensively in electrolyticplating processes, such as zinc-plating, copper-plating, silver-platingetc. processes.

Since for sanitary reasons increasingly higher demands are put upon thepurification of industrial waste water, this has resulted in greattechnical and economical problems. This is illustrated by the fact thatfor depositing one kilogram of metal, normally about 5 cubic meters ofwater are used.

The destruction of the cyanide in the waste water from the electrolyticplant is normally carried out by decomposing it by oxidation, usingchlorine as the oxidizing agent, the chlorine being supplied either asgaseous chlorine or as a hypochlorite. To avoid the formation of thetoxic chlorocyanogen the oxidation must be carried out at a rather highpH value. The cost will therefore be relatively high and normallyamounts to three or four times the cost of the cyanide used.

The object of the present invention is to provide a more economicalsolution of the problem of disposing cyanidecontaining liquids obtainedin chemical processes for surface-treatment of metals. The method of theinvention also provides several other advantages, inter alia a drasticreduction of the consumption of water for the process. The invention isbased on the principle of electrolytically decomposing the cyanide inelectrolytic pretreatment baths.

Electrolytic oxidation has been used to some extent in practice todestroy cyanides. However, due to several practical problems the methodhas found only limited use. One of the great disadvantages has been highinstallation costs.

In the anodic oxidation of cyanide, carbonates and ammonia are formed.The process is accelerated by elevated temperatures and a high pH. Alsothe over-voltage of the anode material for oxygen is important. The bestmaterial is platinum which has the highest oxidation potential. Providedthat the pH is about 11 and the temperature about 70 0., iron and carbonare also satisfactory and a high current yield is obtained in thedecomposition process.

Each electrolytic plating process is normally preceded by anelectrolytic cleaning of the articles in a hot alkaline bath. Such abath provides ideal conditions for an anodic decomposition of thecyanides.

According to the present invention an electrolytic decomposition ofcyanides from surface treatment baths is carried out in one of theelectrolytic process steps, in particular in the electrolytic cleaningbaths.

The invention is illustrated but not limited by the following examplewhich is diagrammatically shown on the accompanying drawing. The examplerelates to electrolytic zinc-plating in cyanide-containing electrolyticbaths.

As shown on the drawing, the installation comprises a sequence of baths1-12 for treating metal articles for zinc-plating. These baths have thefollowing functions:

The metal articles to be treated pass through the baths mainly in theorder mentioned. The thin arrow lines to the right in the figureillustrate the flow of liquid to and from and between the baths asexplained in detail below.

More particularly, the baths may operate in the following manner:

Bath 1 is a degreasing bath containing sodium hydroxide, sodiumcarbonate and metasilicate, operating at a temperature of about C.

Bath 2 is an electrolytic degreasing bath of the same composition asbath 1. The article is connected as a cathode. The anode consists ofgraphite. The bath operates at a temperature of about 80 C. and at acurrent density (anodic and cathodic) of about 5 to 10 amperes per sq.dm.

Bath 3 is also an electrolytic degreasing bath of the same compositionas bath 2 and operated under similar conditions, except that the articleis coupled as the anode and the cathode consists of steel. Suitably, acomplexing agent of gluconate type is added to this bath.

The evaporation of water from baths 1, 2 and 3 (in dicated by theoutwardly directed arrow at bath 1) is about 8 to 10 liters per sq. m.of bath surface an hour. To compensate this liquid loss, and to achievethe desired destruction of cyanides, liquid is passed from bath 4 tobaths 1, 2 and 3 as indicated by the arrows.

Bath 4 is a cold water bath for rinsing the articles after they havebeen treated in bath 3.

Bath 5 is also a cold water bath for further rinsing the articles beforethey are treated in the plating bath. The articles may also be rinsed inthis bath after they have been treated in the plating bath.

Bath 6 is a cyanide-containing electrolytic zinc-plating bath,containing about 35 g./l. Zn, 30 g./l. NaOH and g./l. NaCN. The bathoperates at a temperature of not above 30 C. and at a current density ofabout 3 to 6 amperes per sq. dm.

Bath 7 is a cold water bath for rinsing the electroplated articles,which may previously have been rinsed in bath 5. Fresh water isintroduced into bath 7 at a rate corresponding to the evaporation inbaths 1, 2, 3, the water flowing through baths 7, 5, 4 and thence tobaths 1,2,3 as indicated by the arrows in the diagram on the drawing.

Thus, zinc and cyanide accompany the water to the degreasing baths 3,2, 1. The zinc is precipitated in bath 3 3 on the cathodes. The cyanideis decomposed by electrolytic oxidation in baths 3 and 2. To avoidintroducing cyanide into bath 1 it may be suitable in some cases tointroduce water from bath 4 only into baths 3 and 2 and take water forbath 1 from bath 2 as indicated by the valve and the broken line arrow.

Bath 8 is a cold water bath for further rinsing the article. From thisbath, water is circulated through a basic anion exchange unit 13, whereany residual zinc cyanide complexes are taken up. The ion exchangecomposition is regenerated, when necessary, with NaOH and the regeneratemay be passed to the zinc bath 6.

Bath 9 is an acid pickling bath containing about 1% HNO Bath 10 is aso-called one-step ehromating bath containing about 2 g./l. alkalibichromate and fluoride and sulfate ions. Total salt content about 4g./l.

Bath 11 is a cold water rinsing bath, wherein the water is circulatedthrough a two bed (anionic and cationic) ion exchange filter unit 14, 15so thatthe water will be completely deionized. Fresh water is introducedinto this bath (through the filter unit) to compensate evaporation inbath 12.

Bath 12 is a final hot water rinsing bath operating at about 80 C.Evaporation is about 8 liters per sq. m. an hour, which is replaced bywater taken from bath 11.

We claim:

1. In a method of surface treatment of metal articles incyanide-containing surface treatment baths, wherein thesurface-treatment is preceded by cleaning the articles in electrolyticalkaline cleaning baths and the articles after surface-treatment arerinsed in water, the improve- 1 ment which comprises passingcyanide-containing water from the rinsing baths to said electrolyticcleaning baths,

whereby the cyanide will be submitted to electrolytic oxidation in saidbaths.

References Cited UNITED STATES PATENTS OTHER REFERENCES Disposal ofWaste Cyanides by Electrolytic Oxidation by R. W. Oyler, Plating, April1949, pp. 341-342.

Destruction of Cyanide Copper Solutions by Hot Electrolysis by L. B.Sperry and M. R. Caldwell, Plating, April 1949, pp. 343-347.

JOHN H. MACK, Primary Examiner T. T UFARIELLO, Assistant Examiner US.Cl. X.R. 204-149

